The instant invention concerns a system for control of a CVT driven by an input unit.
Continuously variable automatic transmissions, also called CVT, for motor vehicles are usually comprised of a start-up unit, a forward/reverse drive unit, a variator, an intermediate shaft, a differential and a hydraulic and electronic control unit. The CVT is driven by an internal combustion engine, via an input shaft, particularly the crankshaft. A start-up unit serves either a start-up clutch or a hydrodynamic converter. The forward/reverse drive unit serves for reversal of direction of rotation for forward or reverse drive and usually consists of a planetary reversing gear.
The variator consists of two bevel pulley pairs and a belt-type organ, each bevel pulley pair having a first bevel pulley axially stationary and a second bevel pulley axially movable. Hereinafter the bevel pulleys disposed on the input will be designated as primary pulley and the bevel pulleys disposed on the output as secondary pulley. Between the two pairs of bevel pulleys rotates the belt-type organ such as a sliding link conveyor.
The running radius of the belt-type organ and therewith the ratio of the CVT change by adjustment of the bevel pulleys. The second bevel pulley pair is non-rotatably connected with one output shaft which transmits the torque to an intermediate shaft via a pair of gears. The intermediate shaft serves for reversal of direction of rotation and adaptation of torque and rotational speed. The torque of the intermediate shaft is transmitted to the differential via one other pair of gears.
The control or regulation of the CVT is carried out by the electronic control unit via the hydraulic control unit. The hydraulic control unit contains electromagnetic actuators and hydraulic valves. A pump conveys the pressure medium from the lubricant sump to the hydraulic control unit and to the consumers of the CVT.
EP-A 0 634 590 has disclosed a system for hydraulic control of a CVT. The primary pulley and the secondary pulley are each adjusted via a hydraulic valve. The first valve controls the primary pulley and the second valve controls the secondary pulley. Both the first and second valves are pre-controlled by an electromagnetic pressure regulator. The second valve is here supplied with pressure medium by a pump, the pressure supply of the first valve being adjusted via a control edge of the second valve. The pressure level of the supply line for the first valve is thus identical with the pressure level delivered by the pump. The disadvantage resulting from this arrangement is that the pressure level of the secondary pulley cannot be adjusted below the minimal pressure level provided by the pump so that an overpressure between the secondary pulley and a belt-type organ results in a reduction in efficiency.
To overcome said disadvantage, the applicant's patent application 195 33 976.2 has already proposed a hydraulic system for a CVT preferably driven by an internal combustion engine having a common central first hydraulic branch in which a first pressure level prevails and which is fed by a pump from which a specific pressure supply branches off from the first hydraulic branch to each consumer; the consumers are here the primary pulley, the secondary pulley, the clutch or brake of the forward-reverse drive unit, the start-up unit and the lubrication points of the CVT. In each pressure feed to a consumer are located both a hydraulic valve and an electromagnetic pressure-control valve associated therewith in a manner such that the electronic pressure-control valve determines via the hydraulic valve the pressure level in the pressure feed of the consumer. The electromagnetic valves are, in this case, situated in a second hydraulic branch.
From the applicant's DE-P 44 36 506.3 is known a system for control of a CVT driven by an input unit which has a so-called emergency device, with two emergency valves, which is activated in case of failure of the electronic control unit and which provides a constant pressure ratio or power ratio between primary pulley and secondary pulley, the pressure level of an adjusting chamber of the secondary pulley then being constant. The first emergency valve is connected with the primary valve for the primary pulley and the secondary emergency valve with the secondary valve for the secondary pulley; the shift positions of both emergency valves are determined by an electromagnetic pressure-control valve in a manner such that in a first position of both emergency valves a first pressure-control valve acts upon the primary valve pre-controlling it and a second pressure-control valve acts upon the secondary valve pre-controlling it while in a second position of both emergency valves the pressure level of a pressure-reducing valve acts upon the primary valve and the secondary valve pre-controlling them.
The problem to be solved by the present invention is to propose a system for control of a CVT which offers the same advantages and reliability as the systems already known but, at the same time, simplifies construction and thus more economical production.